Biodiesel is a type of biofuel that is manufactured from triglycerides, diglycerides, and monoglycerides, but predominantly triglycerides. Vegetable oils, nut oils, animal fats, seed oils, fish oils, and the like are examples of suitable feedstocks containing triglycerides. In a typical synthesis, triglycerides are subjected to a transesterification reaction between the triglyceride and a stoichiometric excess of a suitable alcohol such as methanol, ethanol, or other linear, branched or cyclic C4, C5, or C6 alcohols. Use of ethanol and methanol are most common. The reaction occurs in the presence of a base catalyst and usually under substantially anhydrous conditions in which water is excluded as much as is practical. The reaction may be carried out in continuous or batch equipment.
The desired product of the transesterification is a fatty acid ester. When the alcohol reactant is methanol, this product is referred to as a fatty acid methyl ester, or FAME. The final contents of the reaction stream will also include glycerin (also known as glycerol) as a by-product alcohol; unreacted excess reactant alcohol; residual and spent catalyst (the spent catalyst may be present as a soap depending upon the catalyst used); and soaps present from fatty acids or other impurities that might have been present in the oil feedstock. The reaction usually proceeds far enough to completion that the amount of glyceride (whether mono, di, or tri) is de minimis.
The by-product glycerin is insoluble in the product ester to a large degree. Accordingly, the reaction stream separates into two phases as the transesterification reaction progresses. One phase is relatively rich in the fatty acid ester, while the other phase is relatively rich in glycerin. All of the constituents of the reaction vessel tend to be distributed among both phases, however. The glycerin layer is referred to herein as “crude glycerin”. The other organic ingredients of the crude glycerin layer are referred to herein as contaminants with respect to the crude glycerin.
Glycerin itself is a triol having the formula HOCH2CH(OH)CH2OH and has many uses. By way of example, it is used in medical and nutriceutical preparations, in personal care products, in foods and beverages, in animal feed, as a raw material to manufacture other compounds such as polyols and polyurethanes, in surface coatings and paints, in making absolute ethanol, in textiles, in de-icing fluids, in softeners and surfactants, in antifreeze, and the like. Accordingly, it is highly desirably to purify the crude glycerin inasmuch as glycerin has so many product uses. The methanol, fatty acid, and fatty acid ester contaminants in the crude glycerin also are valuable materials and are desirably recycled as well. For instance, the methanol, fatty acid, and the fatty acid ester can be recycled for use in further synthesis of biofuel or other products.
A key step in the purification of crude glycerin involves stripping the methanol from the crude glycerin using distillation techniques. Conventional methodologies have been problematic, however. In some instances, the distillation occurs under substantially anhydrous conditions. However, relatively high bottom temperatures must be used, e.g. temperatures above about 200° C., even above about 210° C., and even above about 220° C., in order to reduce the methanol content of the crude glycerin to acceptably low levels when distillation is anhydrous. At these temperatures, there is a substantial tendency for undue amounts of polyglycerin to form, undermining the goal to obtain purified glycerin. Temperature reduction by operation under vacuum to lower the temperatures requires a more sophisticated condenser/cooling system.
Carrying out wet distillation, however, is also problematic. Often, decanted wash water might be added to the crude glycerin in order to recover more fatty acid ester in an organic phase, which segregates as an upper layer on top of the glycerin. Methanol stripping from this or any other similarly wet layer is difficult due to excessive foaming caused by soap that is present. There is too much water, soap, and foaming for anti-foaming agents to help control this in any effective manner.
To attempt to avoid foaming, the crude, wet glycerin can be acidified to lower the pH to a value such as 2 to 5 in order to convert the soap into fatty acid. Still, the stripping of methanol from such acidic glycerin poses serious challenges due to corrosivity and reboiler plugging issues. First, the crude glycerin is corrosive due to its low pH, requiring equipment with expensive metallurgy for proper handling. Reboiler plugging can be caused by salts and the high distillation efforts to separate a dry methanol from such a wet glycerin. Reboiler plugging is a severe economic issue. The heat transfer coefficient decreases and the unit loses production capacity over time. Eventually, the unit will have to be shut down to remove salts by washing them out, by hydro blasting, or other suitable technique.
U.S. Pat. Nos. 6,262,285; 6,174,501; 7,126,032; and 7,138,536; as well as JP 10218810, discuss glycerin purification following biofuel synthesis.
There remains a strong need for effective methodologies that can purify crude glycerin, including aspects of this purification that involve separating crude glycerin from other alcohol contaminants such as methanol.